This invention relates to the forming of readable precision structures, a pattern, mark, or other indicia on metrological and like equipment by irradiation with a beam of energy. In particular but not exclusively it relates to the formation of a pattern of marks on an object e.g. graduation structures on a scale to be used for metrological purposes, such as in an optical encoder.
FIG. 11a is a diagram illustrating in detail certain lengthwise topography and pertinent dimensions of an example scale mark;
Optical encoders typically employ scales consisting of a series of spaced-apart marks on a substrate. In the case of a reflective scale, the scale may have reflective marks formed on a non-reflective background, or vice-versa. Transmissive scales may have opaque marks formed on a transparent substrate, or vice-versa. Both reflective and transmissive scales interact with light from an optical source (such as a light-emitting diode (LED) or laser) to create an optical pattern that can be detected by an optical detector. As relative movement between the scale and the detector occurs, the optical pattern changes in a corresponding fashion, and the detector and associated electronic circuitry translate the pattern changes into precise numerical position indications. Scales of the above type, which are known as “amplitude” scales, have been manufactured in a variety of ways, including a commonly used approach of forming the scale marks as metal traces on a glass substrate.
U.S. Pat. No. 5,632,916 discloses a laser method of producing optically readable marks on a metal surface such as a machine part. It is noted that prior marking methods have included (a) engraving on the surface (melting, vaporization) and (b) providing a chemical reaction/change in microstructure by means of a laser (heating), and that the engraving techniques are characterized in that they break the original surface and are situated at a level lower than the original surface. An object of the invention in '916 is to provide a method of producing optically readable marks on a metal surface in such a way that the metal surface is substantially even and has a very good abrasion and corrosion resistance in spite of the marking.
In the laser method disclosed in the '916 patent, a beam of an excimer laser is used to form areas on a chromed metal surface that are discernible as darker areas from the surrounding reflective metal surface. The metal surface is exposed to a laser beam pulse, the energy of which is 1 to 10 J/cm2, preferably 3 to 5 J/cm2 and a duration 5 ns to 1 us, preferably 15 to 30 ns. A spot of impact of the laser beam pulse on the metal surface is changed in such a way that a new spot of impact overlaps previous spots of impact and the metal surface is exposed to a new laser beam pulse, such that an area of the metal surface gets a color contrasting with the original metal surface. The effect of different marking parameters on the darkness (contrast) of a mark and on the surface roughness was studied in a test. In one embodiment, to provide a preferably uniform colored area, the propagation of the edge of successive pulses is 0.1 mm or less. The operating range of the excimer laser, i.e. the repetition frequency range of the pulses, is about 1 to 400 Hz. There is no disclosure in the '916 patent of the spatial distribution the spot/beam intensity at the metal surface.
In one example disclosed in the '916 patent, a hard chrome surface was ground after the chroming in such a way that the value of surface roughness Ra was 0.2 μm or better. The metal surface was moved with respect to the beam in such a way that the propagation of the metal surface between successive pulses was between 0.020 mm and 0.012 mm. The width of the beam on the metal surface was 1 mm and the height in the motion direction varied between 0.2 and 2 mm. In the example shown in the figures, the surface layer consists of a chrome plating on the surface of a steel bar. The thickness of this chrome plating is about 30 μm. The thickness of the marked areas is less than 1 μm, such that the surface is substantially smooth and at the same level as the remainder of the metal surface, which is due to the fact that material is not vaporized from this surface, at least not essentially. It is possible that the laser beam pulse removes a very thin oxide layer from the metal surface. The measured surface roughness values show that the marking method does not impair the surface roughness of the hard chrome surface. The surface roughness is approximately the same measured before marking and after marking at a dark line. The effect of atmosphere was also investigated and found to be negligible.
It is known that there are a number of different mechanisms for pulsed laser processing of materials and that the key factor for determining which mechanism is employed is dependent on the result desired and the base material. For example, metals may be chemically reacted, melted, boiled eruptively, sublimated or molecularly disassociated as a function of power and pulse period. Plastics also can be melted chemically reacted, and molecularly disassociated but boiling eruptions are often preempted by charring. By and large these processes operate to some degree in all solid materials by varied wavelength, power and pulse properties.
With pulses in the low nanosecond range, metal behavior can be as follows. At low pulse energy densities the material may oxidize or react with gases in the atmosphere to change color, and/or it may also re-alloy based on the melting points and solubility of the various constituents or chemically react within its constituent materials. With increased pulse energy density the surface may melt and flow as well as small amounts of plasma may generate and escape. As more material is engaged the materials may begin to violently boil and eject large volumes of material leaving a chaotic craters-and-debris field. If the laser intensity and associated field strength are high enough, as may occur with high intensity ultrashort pulses, a nearly instantaneous transition from the solid to vapor state may occur wherein material removal occurs, avoiding formation of slag or debris.
The invention also relates to methods and systems for precision workpiece processing during motion of a workpiece relative to a tool, and more specifically to processing a workpiece using an energy beam, for instance a pulsed laser beam, to form precision patterns on the workpiece during motion. By way of example, the workpiece may be a flexible substrate, and the processing carried out with a laser beam to produce a flexible metrological tape scale with graduations having optical contrast.
In US Published Patent Application 2005/0045586 (hereafter referred to as '5586) it was disclosed that the production of measurement scale using a laser light to mark its surface has been considered previously. It was noted that In U.S. Pat. No. 4,932,131 an in-situ scale writing or calibration technique is used. A reference is used to lay-down marks or correct any deficiencies in the scale. A laser is used to read and write a scale. In the '5586 it was indicated that the '131 patent does not disclose the method for doing this, and has no mention of overcoming thermal problems.
'5586 also discloses a method of producing precision marks for a metrological scale, employing apparatus including: a scale substrate to be marked at repeated instants by a laser and thereby forming a metrological scale; a laser operable so as to provide light pulses for forming scale markings at the substrate; a displacement device for causing relative displacement between the substrate and the location at which the light is incident on the substrate; and a controller for controlling the relative displacement and the laser, the method comprising the steps, in any suitable order, of: operating the displacement mechanism so as to cause relative displacement between the substrate and the light; using the controller to control the relative displacement and to operate the laser so as to produce light pulses at the substrate; characterized in that: the laser produces a plurality of ultra-short output pulses of a fluence at the substrate such that the metrological scale marks are formed by laser ablation.
The '5586 publication also discloses a laser light manipulation device, a displacement sensor for sensing the displacement between the substrate and the location at which the light is incident and a reader for determining the distance between two or more markings at the scale wherein the method further comprises: issuing a signal from the displacement sensor to the controller; issuing a signal from the reader to controller; in response to the signals from the sensor and the reader using the controller to control the manipulation device, the displacement, and the repeated instants at which the laser ablates the substrate.
FIG. 2 of the '5586 patent shows two pinch rollers 20 and 22 used to feed the ribbon (which has constant tension through the station 100). Pinch roller 20 is driven at an approximately constant rate but no speed governing need be used other than a controllable voltage supply. Pinch roller 22 has two rotary encoder rings 24 affixed thereto or marked thereon. Two readers 26 read the encoder markings to provide a machine controller (200 FIG. 1) with two signals so an average of the two can be used to provide a ribbon displacement value to the controller. This averaged ribbon displacement signal at the machine controller 200 is used, via software, to govern the firing of ribbon marking laser 21.
Additionally, '5586 discloses a system of two or more scale readers, in this instance two readers 23a and 23b, are used to read the scale that is being produced by the laser 21. The readers 23 are set at a pre-determined distance L apart and so any errors in the pitch of the markings can be determined and adjustment via software of the laser firing rate can be made if appropriate. So even if the temperature at the laser irradiated area increases slightly, the temperature at the readers will remain constant and then any slight heating by the laser light can be compensated for by increasing the scale pitch at the laser irradiated area.
Japanese Patent 5169286 (based on certified translation received), also cited in '5586, shows a method of obtaining a marking perpendicular to the direction of travel of a measurement scale which is being marked using a laser. As shown in the drawing, the system includes: a laser oscillator, a deflecting mirror, an f-θ lens, an actuator, a moving device, a moving table, a motor, encoder, scale member, first controller, second controller. Included is a device for marking a scale line by irradiating the front surface of a member to be scaled that is carried in the scale direction at a constant speed with a laser beam swept by a scanner, that is equipped with a controller that operates the scanner such that the location that is irradiated by the laser beam moves in a direction forming angle θ with the direction of the carrying speed V of the member at a speed of V/cos θ. The system is equipped with a controller that operates the scanner such that the location that is irradiated by the laser beam moves in a direction forming angle θ with the direction of the carrying speed V of the member at a speed of V/cos θ, starts the laser beam irradiation at a point in time corresponding to the starting position of the scale line to be marked, and finishes this laser beam irradiation after an amount of time corresponding to the length of the scale line has elapsed. The starting point of the laser beam irradiation corresponds to a position signal that is output when the member to be scaled reaches a prescribed position. The location that is irradiated by the laser beam moves in a direction forming angle θ with the direction of the carrying speed V of the member to be scaled at a speed of V/cos θ, so scale lines are marked on the member to be scaled in a direction orthogonal to the scale direction, the direction in which the laser beam is swept by deflecting mirror is not orthogonal to the scale direction, but instead forms a prescribed angle as described later, and that a second controller is used in place of the first controller. Moreover, this embodiment differs operationally from the example of prior art in that: scale plate 9 is successively carried at a constant speed; a prescribed relationship is established between the carrying speed of the scale plate, the speed at which deflecting mirror is swept, and the sweeping direction; and the manner in which the timing of the laser beam emission is determined is predefined.
U.S. Pat. No. 5,741,381 describes a labeling system and method. FIGS. 8 and 10 show a radius sensor that provides radius data to a processor to translate rotational speed of a motor drive and a radius of a roll into a linear feed rate. An alternative optical sensor reads registration indicia.